Mobile radio communication system and fixed and mobile units used therefor

ABSTRACT

In a mobile communication system of the present invention, a cordless base unit LBU is connected to a wired subscriber line SL. The base unit LBU detects a radio channel assigned to a cell other than a cell where the base unit LBU is located, among radio channels assigned to the cells of a cellular radio communication system. This radio channel is used as a radio channel for a cordless call. On the other hand, a cellular cordless telephone CCT is provided with a mode switch. When a cordless mode is designated by the mode switch, a radio channel for a cordless call is detected, as with the case of the base unit LBU. At the time of the cordless call, a transmission power level of the cellular cordless telephone CCT is set at a weak level smaller than that for a cellular call.

TECHNICAL FIELD

The present invention relates to a mobile communication system obtainedby improving a cellular type radio communication system.

BACKGROUND ART

For example, a cellular type automobile/portable radio telephone systemhas been known as one of mobile communication systems.

For example, as shown in FIG. 9, this type of system comprises a controlstation CS connected via a wired line CL to a stational exchanger EX ofa public station network, a plurality of base stations BS1 to BSnconnected via wired lines CL1 to CLn to the control station CS, and aplurality of mobile stations MS1 to MSm. The respective base stationsBS1 to BSn constitute radio zones E1 to En called cells in differentareas within a service area of the system. Different radio channelgroups are assigned in advance to the radio zones E1 to En. These radiochannel groups are repeatedly assigned to other cell groups located at asufficient distance, thereby effectively utilizing the radio channels.FIG. 10 shows an example of this structure, and radio channel groups F1to F13 are assigned to 13 cells constituting each cell group.

Regarding the mobile stations MS1 to MSm, each station is connected tothe base station of the radio zone, in which the present station islocated, via an unoccupied radio channel of the radio channel groupassigned to this radio zone, and the base station is in turn connectedto the stational exchanger EX via the control station CS. In this state,a call can be made with a wired telephone connected to the stationalexchanger EX or another mobile station. If the mobile station, MS1 toMSm, has moved to another radio zone during a call, the radio channel isswitched to a radio channel assigned to the radio zone of thedestination of the mobile station and the call is continued.Specifically, as long as the mobile stations MS1 to MSm are presentwithin the service area of the system, a call can be made between eachstation and another telephone via the base station of the radio zone(cell) where the station is located.

This cellular radio telephone system, however, has the followingproblem. In the cellular radio telephone system, a mobile station ispermitted to make a call anywhere in the service area. Thus, when theuser is present, in particular, outdoors, the feature of the system canbe exhibited to a maximum. However, the charging rate of the call fee ofthe cellular radio telephone system is generally higher than that of thepublic telephone network using wired lines. Accordingly, when the usermakes a call in the home or office, it is economical to use a wiredsubscriber line of the public telephone network, rather than using thecellular radio telephone system. For this purpose, however, the usermust deliberately choose between the use of the mobile station of thecellular radio telephone system, i.e. the portable telephone, and theuse of the wired telephone installed in the home or office, each time hemakes a call. This is very troublesome.

DISCLOSURE OF INVENTION

A first object of the present invention is to provide a mobilecommunication system capable of performing cellular communication andcordless communication with one mobile unit.

A second object of the present invention is to provide a mobilecommunication system capable of more effectively utilizing a radiochannel assigned to a cellular radio communication system.

A third object of the present invention is to provide a mobilecommunication system which does not incur an increase in size and weightof a mobile unit, does not cause a considerable increase in cost, andcan be realized easily.

In order to achieve the first and second objects, the mobilecommunication system of the present invention comprises a cellular radiocommunication system and a fixed unit. In the cellular radiocommunication system, a predetermined physical area is divided into aplurality of cells, and radio communication is performed between a baseunit provided in the cell and a mobile unit moving in the physical area.For this radio communication, the radio channel assigned to the cellwhere the mobile unit is located is used. The transmission output levelof the mobile unit is set in a predetermined first transmission outputlevel range. The fixed unit is connected to a public communicationnetwork via a wired subscriber line.

The fixed unit and the mobile unit comprise means for performing radiocommunication between these units. The radio communication meansperforms radio communication between the fixed unit and the mobile unitby using the radio channel which is assigned to any of the cells of thecellular radio communication system and is other than the radio channelassigned to the cell where the mobile unit is located, at a level in asecond transmission output level range smaller than the firsttransmission output level range.

According to this structure, only by buying a mobile unit and a fixedunit for cordless communication, can the user make a cellular call, e.g.in the outdoors, with the mobile unit by making use of a cellular radiocommunication system. Besides, in the home or office, the user can makecordless communication by making use of the wired subscriber line. Thus,with the portability of a cellular terminal maintained, the call fee canbe economically reduced. Furthermore, the user does not need toselectively use the portable telephone for the cellular radiocommunication system or the wired telephone installed in the home oroffice. Only by using a single mobile unit can the user make cellularcommunication or cordless communication.

In the mobile communication system of the present invention, the fixedunit comprises first receiving means, second receiving means anddetection means. The second receiving means receives a signal from themobile unit. The first receiving means selectively receives a signalsent via the radio channel assigned to each cell of the cellular radiocommunication system. The detection means determines whether the radiochannel other than the radio channel assigned to the cell where thefixed unit is located, among the radio channels assigned to the cells ofthe cellular radio communication system, is available on the basis ofthe signal received by the first receiving means.

According to this structure, a radio channel with less interference isselected from the radio channels possessed by the cellular radiocommunication system, and cordless communication is performed betweenthe mobile unit and fixed unit via the radio channel. Thus, theefficiency of use of the radio channels possessed by the cellular radiocommunication system can be enhanced, and there is no need to assign anew radio channel for cordless communication. Therefore, this techniquecan easily be practiced.

In order to achieve the third object, the mobile communication system ofthe present invention has one receiving means in the mobile unit. Thereceiving means receives, in a first time period, a signal sent via aradio channel designated by the base unit, and receives, in a secondtime period different from the first time period, a signal sent via theradio channel which is assigned to any of the cells of the cellularradio communication system and is other than the radio channel assignedto the cell where the mobile unit is located. Based on the signalreceived by the receiving means, a call coming from the base unit and acall coming from the fixed unit are detected.

According to this structure, in the mobile unit, one receiving meansreceives the signal from the base unit of the cellular radiocommunication system and the signal sent from the fixed unit in atime-sharing manner. Thus, only one receiving means may be provided. Ascompared to the case where, for example, the mobile unit is providedwith a structure for cellular communication and a structure for cordlesscommunication, the circuit configuration of the mobile unit can besimplified and the unit can be reduced in size and weight.

In the mobile communication system of the present invention, the mobileunit is provided with operation means. The operation means is used todesignate/input the base unit or the fixed unit as a radio connectiondestination. Based on the designated/input result of the operationmeans, a radio communication operation with the base unit and a radiocommunication operation with the fixed unit are selectively performed.

Furthermore, in the mobile communication system of the presentinvention, a signal sent from the fixed unit is monitored in the mobileunit. While a signal sent from the fixed unit to the mobile unit isbeing received, a cordless mode is set. In the other time period, acellular mode is set. In accordance with the set mode, a radiocommunication operation between the mobile unit and the base unit and aradio communication operation between the mobile unit and the fixed unitare selectively performed.

According to these structures, the mobile unit may be provided with onlyone transmission/reception means. Thereby, the circuit configuration ofthe mobile unit can be simplified and the unit can be reduced in sizeand weight.

On the other hand, the fixed unit of the present invention includes, inaddition to wired communication means for signal transmission/receptionwith a public communication network via wired subscriber lines,detection means, radio communication means for radio communication withthe mobile unit of the cellular radio communication system, andcommunication control means. The detection means detects whether theradio channel other than the radio channel assigned to the cell wherethe fixed unit is located, among the radio channels assigned to thecells of the cellular radio communication system, is available. Based onthe detection result, the radio channel to be used by the radiocommunication means is selected and signal transmission/reception withthe mobile unit is performed. If communication is performed between thepublic communication network and the mobile unit is performed, thecommunication control means connects the wired communication means andthe radio communication means.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically the structure of a mobile communicationsystem according to an embodiment of the present invention;

FIG. 2 is a circuit block diagram showing the structure of a cordlessbase unit LBU of the system as shown in FIG. 1;

FIG. 3 is a circuit block diagram showing the structure of a cellularcordless telephone CCT of the system as shown in FIG. 1;

FIG. 4 is a flow chart illustrating the first half of an activationcontrol procedure and the content of control by a control circuit of thecordless base unit LBU as shown in FIG. 2;

FIG. 5 is a flow chart illustrating the second half of the activationcontrol procedure and the content of control by the control circuit ofthe cordless base unit LBU as shown in FIG. 2;

FIG. 6 is a flow chart illustrating a cellular wait control procedureand the content of control by a control circuit of the cellular cordlesstelephone CCT as shown in FIG. 3;

FIG. 7 is a flow chart illustrating a mode switch control procedure andthe content of control by the control circuit of the cellular cordlesstelephone CCT as shown in FIG. 3;

FIG. 8 is a flow chart illustrating a cordless wait control procedureand the content of control by the control circuit of the cellularcordless telephone CCT as shown in FIG. 3;

FIG. 9 shows schematically the structure of an example of a cellularradio communication system; and

FIG. 10 shows an example of assignment of radio channels to therespective cells of the cellular radio communication system.

BEST MODE OF CARRYING OUT THE INVENTION

A mobile communication system according to an embodiment of the presentinvention will now be described.

The system of this embodiment is based on a cellular radio communicationsystem to which the AMPS system prevailing in the U.S.A., etc. isapplied.

A frequency band of, e.g. 824 MHz to 894 MHz is assigned to the cellularradio communication system. In this frequency band, 824 MHz to 849 MHzare used for upward channels from mobile stations to a base station, and869 MHz to 894 MHz are used for downward channels from the base stationto the mobile stations. The number of the upward channels and the numberof the downward channels are, respectively, 832. The present systemadopts a multi-channel access system for each cell. Of the 832 channels,790 channels are assigned to calls, and the other 42 channels areassigned to control.

FIG. 1 shows schematically the structure of the system of the presentembodiment. The same elements as shown in FIG. 9 are denoted by likereference numerals.

A plurality of cellular base stations BS1, BS2, . . . BSi aredispersedly provided within a service area of the present system. Thesecellular base stations BS1, BS2, . . . BSi are connected to a controlstation CS via wired lines CL1, CL2, . . . CLi. The control station CSis accommodated in a stational exchanger EX of a public telephonenetwork via a wired line group CL. The cellular base stations BS1, BS2,. . . BSi constitute radio zones E1, E2, . . . Ei called cells,respectively. In these radio zones (cells) E1, E2, . . . Ei, thirteencall channel groups F1 to F13, which are obtained by dividing theabove-mentioned 790 call channels in units of a predetermined number ofchannels, are assigned in advance in a fixed manner to each unit of, forexample, 13 cell groups, as shown in FIG. 10.

In general, wired subscriber lines accommodated in the stationalexchanger EX are pulled in each home and office, whereby calls can bemade with use of wired telephones.

In the system of the present embodiment, a cordless base unit LBU isconnected to a terminal end of the wired subscriber line SL in parallelto a wired telephone (not shown) or independently. In each cell E1, E2,. . . Ei; and, a plurality of cellular cordless telephones CCT1, CCT2, .. . CCTi are used. These cellular cordless telephones CCT1, CCT2, . . .CCTi are provided with a cellular mode in which radio calls are made viathe cellular base stations BS1, BS2, . . . BSi of the cells E1, E2, . .. Ei and a cordless mode in which radio calls are made via the cordlessbase unit LBU.

The cordless base unit LBU is connected to the associated cellularcordless telephone CCT via a radio call channel. Thereby, the cellularcordless telephone is connected to the stational exchanger EX via thewired subscriber line SL to permit a call. The radio call channel usedto connect the cordless base unit LBU and cellular cordless telephoneCCT is a radio channel selected from the radio channel group which isassigned to each cell E1, E2, . . . Ei of the cellular radiocommunication system and is other than the radio channel group assignedto the cell (cell E1 in FIG. 1) in which the cordless base unit LBU andcellular cordless telephone CCT are located. The selection of the radiochannel is made by searching the radio channel at the cordless base unitLBU and cellular cordless telephone CCT.

FIG. 2 is a circuit block diagram showing the structure of the cordlessbase unit LBU. The cordless base unit LBU comprises a first receivingsystem 10, a cordless transmitting/receiving system 20 having a secondreceiving system, and a control system 40.

The first receiving system 10 receives a radio signal sent from thecellular base station BS1, BS2, . . . BSi and comprises an antenna 11, areceiving circuit (RX1) 12, a frequency synthesizer (SYN) 13 functioningas a local oscillator, and a carrier detection circuit (C-DET) 14. Adetection result of a carrier by the carrier detection circuit 14 isinput to the control circuit 40. The oscillation frequency of thefrequency synthesizer 13 is set by radio channel indication data outputfrom the control circuit 40.

On the other hand, the cordless transmitting/receiving system 20transmits/receives a radio signal to/from the cellular cordlesstelephone CCT. A radio signal sent from the cellular cordless telephoneCCT is received by an antenna 21 and input to a receiving circuit (RX2)23 via a duplexer (DUP) 22. The receiving circuit 23 mixes the radiosignal with a reception local oscillation signal generated from afrequency synthesizer (SYN) 24 and converts it to an intermediatefrequency signal, and further performs, e.g. FM demodulation toreproduce a reception call signal. The reception call signal is sent tothe wired subscriber line SL via a hybrid circuit 25 and a line switch26.

A call signal coming from the stational exchanger EX via the wiredsubscriber line SL is input to a transmission circuit (TX) 29 via a lineswitch 26 and a hybrid circuit 25. The transmission circuit 29frequency-modulates the intermediate frequency signal with the sentsignal and mixes the modulated intermediate frequency signal with atransmission local oscillation signal generated from the frequencysynthesizer 24, thereby converting it to a radio frequency correspondingto a downward radio channel. The radio frequency signal is amplified toa predetermined second transmission level by a transmission poweramplifier. The second transmission level is set to be adequately lowerthan a first transmission level used for a cellular call at the cellularbase station BS or cellular radio telephone. For example, it is set at aweak level used in a general cordless telephone. The radio signal outputfrom the transmission circuit 29 is supplied to the antenna via theduplexer 22 and transmitted to the cellular cordless telephone CCT fromthe antenna 21.

The control circuit 40 includes, for example, a microcomputer as a maincontrol unit and has wireless channel search means 41 and cordlesscommunication control means 42 as its functions. Reference numeral 31denotes a power supply circuit which rectifies a commercial power output(AC100V) and produces a predetermined DC operational voltage Vcc.

The wireless channel search means 41 operates upon turning-on of a powerswitch 32. The radio channels used in the cellular radio communicationsystem are successively received by the first receiving system 10, andthe radio channel used in the cell where the cordless base unit LBU islocated is recognized on the basis of the presence/absence of theassociated carrier. On the basis of the result of recognition, a radiocall channel not used in the cell where the cordless base unit LBU islocated is selected from among the radio call channels (790 channels)assigned to the cellular radio communication system, and the selectedone is stored as a radio channel for a cordless call.

When the cordless communication control means 42 has recognizedreception of a call from the stational exchanger EX on the basis of adetection signal from a call reception detection circuit (16 Hz DET) 28,it constitutes a radio link of the cordless call radio channel betweenitself and the associated cellular cordless telephone CCT, therebypermitting a cordless call by the cellular cordless telephone CCT. Whena call transmission request has come from the associated cellularcordless telephone CCT, the cordless communication control means 42constitutes a radio link of the cordless call radio channel betweenitself and the cellular cordless telephone CCT, thereby permitting acordless call by the cellular cordless telephone CCT.

FIG. 3 is a circuit block diagram showing the structure of the cellularcordless telephone CCT. In the figure, a radio signal sent from thecellular base station and cordless base unit LBU is received by anantenna 51 and input to a reception circuit (RX) 53 via a duplexer (DUP)52. In the reception circuit 53, the radio signal is mixed with areception local oscillation signal generated from a frequencysynthesizer (SYN) 54 and converted to an intermediate frequency signaland then frequency-modulated to be reproduced as a reception callsignal. The reception call signal is supplied to a loudspeaker 55serving as a receiver via a call reception circuit (not shown) andoutput from the loudspeaker 55.

Reference numeral 56 denotes a carrier detection circuit (C-DET) whichdetects the presence/absence of the carrier of the radio channel andtells the detection result to a control circuit 70.

On the other hand, a transmission speech signal input to a microphone 57serving as a transmitter is input to a transmission circuit 58 via atransmission speech circuit (not shown). The transmission circuit 58comprises an FM modulation circuit (MOD) 59 and a transmission poweramplifying circuit 60. In the FM modulation circuit 59, a transmissionintermediate frequency signal is frequency-modulated by the transmissionspeech signal and converted to a radio frequency corresponding to theradio call channel. The transmission power amplifying circuit 60 isconstituted by a variable gain amplifier and the gain thereof is set bya command from the control circuit 70. According to the gain, the radiosignal is amplified to a predetermined level and output. The radiosignal output from the transmission power amplifying circuit 60 isdelivered to the antenna 51 via the duplexer 52 and is transmitted tothe cellular base station BS or cordless base unit LBU from the antenna51.

The control circuit 70 includes, for example, a microcomputer as a maincontrol unit and has, as its control functions, cellular communicationcontrol means 71 functioning in the cellular mode and cordlesscommunication control means 72 functioning in the cordless mode.

The cellular communication control means 71 monitors a call receptionfrom the cellular base station BS and also monitors the operation of acall transmission switch 74 in the wait state in which the cellular modeis being set. When a call reception or a call transmission has occurred,the control means 71 constitutes a radio link of the radio call channelbetween itself and the cellular base station BS, thus permitting acellular call. In addition, the control means 71 monitors a mode switchin the wait state. When the mode switch 75 is operated, the operationmode is switched to the cordless mode.

When the mode switch 75 is operated and the mode is switched to thecordless mode, the cordless communication control means 72 searches aradio channel for use in the cordless call and stays in the wait state.If a call reception notice has come form the cordless base unit LBU orthe call transmission switch 75 is operated in this state, the controlmeans 72 constitutes a radio link of the radio call channel for acordless call between itself and the cordless base unit LBU, thuspermitting a cordless call.

Numeral 73 denotes dial keys for inputting dial information.

The operation of the system having the above structure will now bedescribed.

While the user is being out of his home, he sets its own cellularcordless telephone CCT in the cellular mode. In the cellular cordlesstelephone CCT in this cellular mode, the control circuit 70 monitors thecoming of the call reception signal from the cellular base station BSand the operation of the call transmission switch 74 in steps 6a and 6bin FIG. 6. In this state, if the call reception signal has come from thecellular base station BS, the radio channel is captured according to thechannel indication data from the base station BS and a radio link ofthis radio channel is formed with the cellular base station BS.Subsequently, control for cellular communication is effected.

During the cellular communication, the control circuit 70 supplies again control signal to the transmission power amplifying circuit 60 forthe purpose of the cellular communication. Thus, the gain of thetransmission power amplifying circuit 60 is set at a value correspondingto a predetermined transmission level determined by the cellularcommunication system according to the gain control signal. Thereby, theradio transmission signal is amplified to a necessary and fulltransmission level for reception by the cellular base station BS inaccordance with the distance from the cellular base station BS, and theamplified signal is transmitted.

If the user has come back to his home or office, he turns on the powerswitch 32 of the cordless base unit LBU and operates the mode switch 74of his own cellular cordless telephone (e.g. CCT3 in FIG. 1) to switchthe mode to the cordless mode.

Then, in step 4a in FIG. 4, the control circuit 40 of the cordless baseunit LBU initializes the respective units. Thereafter, the radio channelassigned to the base station BS1 is searched.

Specifically, the control circuit 40 selects one of radio channelsassigned to the cellular radio communication system, and sets a localoscillation frequency corresponding to the frequency synthesizer 13 ofthe first receiving system 10 so as to receive the frequency fch of thisselected radio channel in step 4b. Thus, the first receiving system 10performs the reception operation of the radio channel frequency fch. Thecarrier detection circuit 14 detects the result of reception, i.e., thepresence/absence of the carrier and transmits the result of detection tothe control circuit 40. In step 4c, the control circuit 40 determineswhether the radio channel is a radio channel used by the base stationBS1 on the basis of the presence/absence of the carrier. If the radiochannel is that being used, the control circuit 40 determines in step 4dthat this radio channel is the radio channel assigned to the cell E1 inwhich the cordless base unit LBU is present and stores it in the memory.The control routine then advances to step 4e. On the other hand, if theradio channel is not being used, the control goes to step 4e. In step4e, it is determined whether all channels assigned to the system havebeen searched. If there is a non-searched channel, a radio channel fchto be received subsequently is selected in step 4f and the controlreturns to the receiving step 4b. Then, the radio channel search controlof steps 4b to 4f is repeated until the channel has been searchedcompletely.

If all channels have been searched, the control circuit 40 provisionallydetermines in step 4g the radio channel for the cordless call on thebasis of the radio channel determined to be in use by the search. Forexample, if radio channels Chm to Chm+n are assigned to the cellularbase station BS1, a radio channel Chm+n+k, which is discrete by apredetermined frequency, is selected from the radio channels Chm toChm+n as a radio channel for the cordless call.

On the other hand, if the mode switch 74 is operated, the cellularcordless telephone CCT3 executes the mode switch control by the controlcircuit 70. FIG. 7 is a flow chart illustrating the control procedureand control content therefor.

Specifically, in step 7a, the control circuit 70 selects one of theradio channels assigned to the cellular radio communication system. Thefrequency fch of this radio channel is received by the reception circuitRX. In step 7b, the use/non-use of the radio channel is determined onthe basis of the presence/absence of the carrier detected by the carrierdetection circuit 56 at this time. If it is determined that the radiochannel is being used, it is determined in step 7c that this radiochannel is that assigned to the cell E1 where the cellular cordlesstelephone CCT3 is located, and this radio channel is stored. The controladvances to step 7d.

On the other hand, if the radio channel is not being used, the controlgoes to step 7d directly. In step 7d, it is determined whether allchannels assigned to the system have been completely searched. If thereis a non-searched channel, the radio channel frequency fch to bereceived subsequently is selected and then the control returns to thereception in step 7a.

Following this, the radio channel search control in steps 7a to 7e isrepeated until the channel has been searched.

If all channels have been searched, the control circuit 70 provisionallydetermines in step 7f the radio channel for the cordless call on thebasis of the radio channel determined to be in use by the search. Forexample, if radio channels Chm to Chm+n are assigned to the cellularbase station BS1, a radio channel Chm+n+k, which is discrete by apredetermined frequency, is selected from the radio channels Chm toChm+n as a radio channel for the cordless call.

Thus, at the cordless base unit LBU and cellular cordless telephoneCCT3, respectively, the radio channel for the cordless call isprovisionally set.

If the radio channel for the cordless call has been provisionally set,the control circuit 40 of the cordless base unit LBU uses the radiochannel Chm+n+k in step 4h, as shown in FIG. 5, and sends identificationinformation LBU-ID of the cordless base unit LBU from the transmissioncircuit 29.

On the other hand, the control circuit 70 of the cellular cordlesstelephone CCT3 receives the radio signal from the cordless base unit LBUwith the provisionally set radio channel Chm+n+k in step 7g, as shown inFIG. 7. While the passing of a predetermined time period is-beingmonitored in step 7i, the reception of identification information LBU-IDfrom the cordless base unit LBU is detected in step 7h. If LBU-ID doesnot come within the predetermined time period, the control circuit 70executes step 7j to increase k and select the next radio channelChm+n+(k+1) and returns to step 7g. Then, the radio signal from thecordless base unit LBU is received with the radio channel Chm+n+(k+1)and the reception of identification information LBU-ID is monitored insteps 7h and 7i.

In this state, suppose that the identification information LBU-ID hascome to the present device from the cordless base unit LBU via a radiochannel. In this case, the control circuit 70 of the cellular cordlesstelephone CCT3 executes step 7k and sends its own identificationinformation CCT-ID via the radio channel which received the LBU-ID. Instep 7l, the radio channel which received the LBU-ID is formally set asa control channel, and the control circuit comes into the cordless waitstate.

On the other hand, when the control circuit 40 of the cordless base unitLBU has sent its own identification information LBU-ID, it detects thereturn of identification information CCT-ID from the cellular cordlesstelephone CCT3 in step 4i, while monitoring the passing of apredetermined time period. If CCT-ID does not come within thepredetermined time period, the control circuit 40 goes to step 41 toincrease k and select the next radio channel Chm+n+(k+1). In addition,after a predetermined time period in step 4m, the control returns tostep 4h and LBU-ID is sent out with the radio channel Chm+n+(k+1).Thereafter, until CCT-ID is returned, the radio channel Chm+n+k isshifted in units of one channel in steps 4h to 4m, and LBU-ID is sentout repeatedly at every predetermined time period.

Besides, suppose that identification information CCT-ID was returnedfrom the cellular cordless telephone CCT3 within a predetermined timeperiod, and the returned identification information CCT-ID coincidedwith the identification information of the cellular cordless telephoneCCT3 corresponding to the pre-registered present unit. In this case, instep 4n, the radio channel Chm+n+k is set as a control channel andthereafter the cordless base unit LBU is set in the cordless wait stateby this control channel.

Thus, radio connection control can be carried out between the cellularcordless telephone CCT3 and cordless base unit LBU via the controlchannel Chm+n+k.

Next, a description will now be given of the case where in this cordlesswait state a call has been made to the cellular cordless telephone CCT3from a wired telephone connected to the public telephone network oranother cellular cordless telephone CCT.

In the cordless wait state, for example, in steps 8a and 8b in FIG. 8,the cellular cordless telephone CCT3 monitors the reception of a callreception notice from the cordless base unit LBU and the calltransmission operation by the call transmission switch 74, and at thesame time monitors repeatedly whether a call has come via the cellularradio communication system. On the other hand, the cordless base unitLBU repeatedly monitors the call reception signal from the stationalexchanger EX via the wired subscriber line SL and the incoming of thecall transmission signal from the cellular cordless telephone CCT3.

In this state, suppose that the wired telephone connected, for example,to the stational exchanger EX is unhooked to make a call to the cellularcordless telephone CCT3 and then the telephone number of the cellularcordless telephone CCT3, e.g. "01234578888", was dialed. The stationalexchanger EX performs a call operation for the associated cellularcordless telephone CCT3 on the basis of the telephone number"01234578888" and the stored LBU-ID of the home or office associatedwith the telephone number, e.g. "08802461470."

Specifically, when the cellular cordless telephone CCT3 is locatedoutside the radio area of the cordless base unit LBU, for example, whenthe user is out of the home, the stational exchanger EX determines fromthe aforementioned telephone number "01234578888" that the destinationof the call is the cellular cordless telephone and transfers a callreception notice and the aforementioned telephone number to the controlstation CS. Then, the control station CS recognizes the cell where thecellular cordless telephone CCT3 is located and sends a call receptionrequest and the telephone number to the cellular base station BS1 of thecell E1. The cellular base station BS1 sends call reception signals tothe cellular cordless telephones within the cell E1. If the cellularcordless telephone CCT3 of the call destination responds, a radio linkis formed between the cellular base station BS1 and the cellularcordless telephone CCT3 by using the radio channel assigned to thecellular base station BS1. Thereafter, a call is permitted between thecaller and the user of the cellular cordless telephone CCT3. In otherwords, a cellular call is permitted via the cellular base station BS1.

On the other hand, if the cellular cordless telephone CCT3 is presentwithin the radio area of the cordless base unit LBU, the stationalexchanger EX determines whether the identification information of thecordless base unit LBU is registered such that it is associated with theaforementioned telephone number "01234578888." If this identificationinformation is registered, a call reception signal is sent to the wiredsubscriber line SL corresponding to the cordless base unit LBU. Then, ifthe call reception signal is detected by the call reception detectioncircuit 28 in the cordless base unit LBU of the call destination, thecall reception signal is sent to the cellular cordless telephone CCT3via the set control channel. If the cellular cordless telephone CCT3responds, a radio link is constituted between the cordless base unit LBUand cellular cordless telephone CCT3 via the radio channel used as thecontrol channel or via another available radio channel. Thereafter, acall between the caller and the user of the cellular cordless telephoneCCT3 is permitted. In other words, a call is permitted via the cordlessbase unit LBU and wired subscriber line SL, that is, a cordless call ispermitted.

While the radio connection control and call control for the cordlesscommunication are being effected, the transmission power level of thecellular cordless telephone CCT3 is controlled to a very low levelaccording to a command from the control circuit 70.

As has been described above, in the system of the present embodiment,the cordless base unit LBU is connected to the wired subscriber line. Inthe base unit LBU, a radio channel assigned to the cell other than thecell where the base unit LBU is located is detected from among the radiochannels assigned to the cells of the cellular radio communicationsystem. This radio channel is used as a radio channel for a cordlesscall. On the other hand, the cellular cordless telephone CCT is providedwith the mode switch 74. When the cordless mode has been designated bythe mode switch 74, a radio channel assigned to the cell other than thecell where the cellular cordless telephone CCT is located is detectedfrom among the radio channels assigned to the cells of the cellularradio communication system. This radio channel is used as a radiochannel for a cordless call. At the time of the cordless call, thetransmission power level is set at a weaker level than at the time ofthe cellular call.

Thus, according to the system of the present embodiment, only by buyingthe cordless base unit LBU and connecting it to the subscriber line SL,can the user make a cellular call, e.g. in the outdoors, with thecellular cordless telephone CCT by making use of the cellular radiocommunication system. Besides, in the home or office, the user can makea cordless call by making use of the wired subscriber line SL. Thus,with the parallel use of the wired subscriber line SL, a widecommunication area characterizing the cellular terminal can bemaintained and the call fee can be economically reduced. Furthermore,the user does not need to selectively use the portable telephone for thecellular radio communication system or the wired telephone installed inthe home or office. Only by using a single cellular cordless telephoneCCT can the user make a cellular call or a cordless call easily.

In the system of the present embodiment, a radio channel assigned to thecell other than the cell where the cellular cordless telephone CCT islocated is detected from among the radio channels assigned to the cellsof the cellular radio communication system. This radio channel is usedas a radio channel for a cordless call. In addition, at the time of thecordless call, the transmission power level of the cellular cordlesstelephone CCT is variably set at a low level. Thus, there is no need toprovide a new radio channel for the cordless call, and the efficiency ofuse of the radio channel assigned to the cellular radio communicationsystem can be enhanced. Moreover, there is no adverse affect such asjamming in the cellular radio communication system, resulting inhigh-quality calls.

Furthermore, in the present embodiment, the cellular mode and cordlessmode can be switched by the mode switch 74 in the cellular cordlesstelephone CCT. Thus, the cellular call and cordless call can be madeonly by providing a single transmission/reception circuit. Therefore,the cellular cordless telephone CCT can be simplified and reduced inweight.

The present invention is not limited to the above embodiment. Forexample, in the above embodiment, the cellular cordless telephone CCT isprovided with the mode switch 74, and either the cellular mode orcordless mode is selected by the mode switch 74. However, the selectionof the cellular mode or cordless mode may be automatically effected bystructural modification.

For this purpose, for example, the cellular cordless telephone CCT isdesigned to receive the radio signal from the associated cordless baseunit LBU at the time of wait. While this radio signal is being received,the cordless mode is set. During the other time period, the cellularmode is set.

It is also desirable that the cellular cordless telephone CCT receivethe radio signal from the cellular base station BS and the radio signalfrom the cordless base unit LBU in a time-sharing manner, in the statein which the cellular cordless telephone CCT can receive both the radiosignal from the cellular base station BS and the radio signal from thecordless base unit LBU. Thereby, a single receiving system can detectthe call reception from the cellular base station BS and cordless baseunit LBU.

Furthermore, in the above embodiment, when a call reception occurs tothe cellular cordless telephone CCT, a call reception access via thecellular base station BS and a call reception access via the wiredsubscriber line SL and cordless base unit LBU are performedsimultaneously in parallel. However, these accesses may be performedsuccessively. For example, at first, the call reception access via thewired subscriber line SL and cordless base unit LBU is performed. If alink is not formed after a predetermined time period therefrom, the callreception access via the cellular base station BS is performed. Thus, aninexpensive cordless call via the wired subscriber line SL can be madepreferentially, thereby further saving the call fee. In this case,however, if the cellular cordless telephone CCT does not respond to thecall reception access via the wired subscriber line SL, the connectionwith the cellular cordless telephone CCT requires a longer time. Thus,it is desirable to generate, until the connection, a tone signal or aspeech message indicating to the caller that the call is being accessed.

In the above embodiment, the cordless base unit LBU is provided with thefirst receiving system 10 for receiving the radio signal from thecellular base station and the second receiving system for receiving theradio signal from the cellular cordless telephone CCT. It is possible,however, to integrate these receiving systems, thereby receiving bothradio signals, for example, in a time-sharing manner. Thus, the hardwarestructure of the cordless base unit LBU can be simplified and reduced insize. Accordingly, the cost of the unit can be decreased.

In the above embodiment, the system adopts an analog modulation systemsuch as FM system as modulation system. However, the present inventioncan be applied to systems adopting a digital modulation system such asπ/4 shift QPSK system or a so-called dual mode system in which an analogsystem and a digital system are combined.

Besides, various modifications can be made without departing from thespirit of the present invention with respect to the structures of themobile unit and fixed unit, the radio channel searching method, theradio link setting procedure, the control procedure and control contentby the mobile unit and fixed unit at the time of callreception/transmission, the method of controlling the transmissionoutput level in the mobile unit.

What is claimed is:
 1. A fixed unit situated in a physical area of acellular communication system wherein the physical area is divided intoa plurality of cells, each cell comprising a base unit and beingassigned one of a plurality of communication channel groups employed insaid cellular communication system, such that at least those of thecommunication channel groups which are assigned to adjacent ones of thecells are different from each other, and radio communication isperformed between the base units of the cells and a mobile unit movingin the physical area by using communication channels of thecommunication channel groups, said fixed unit comprising:a wiredcommunication circuit configured for signal transmission/reception witha communication network via a wired line; a control circuit forsequentially receiving signals transmitted via communication channelsincluded in communication channel groups assigned to cells of thecellular communication system, identifying, on the basis of thereception results, the one of the communication channel groups which isassigned to the one of the cells in which a fixed unit is situated,selecting, on the basis of the identification results, those of thecommunication channel groups which are not assigned to the one cell inwhich the fixed unit is situated, and storing the selected communicationchannel groups; a radio communication circuit configured to select acommunication channel to be used, on the basis of the communicationchannel groups stored in said control circuit, and to perform signaltransmission/reception with the mobile unit via the selectedcommunication channel; and a communication control circuit configured toconnect the wired communication circuit and the radio communicationcircuit in the case of performing communication between thecommunication network and the mobile unit.
 2. The fixed unit accordingto claim 1, wherein said radio communication between said base unit andsaid mobile unit is performed at a level in a first transmission outputlevel range.
 3. The fixed unit according to claim 2, wherein said signaltransmission/reception with said mobile unit is performed at a levelwhich is lower than the output levels in the first transmission outputlevel range.
 4. A mobile unit used in a cellular communication systemwherein a physical area is divided into a plurality of cells, each cellcomprising a base unit and being assigned one of a plurality ofcommunication channel groups employed in the cellular communicationsystem, such that at least those of the communication channel groupswhich are assigned to adjacent ones of the cells are different from eachother, and radio communication is performed between the base units ofthe cells and a mobile unit moving in the physical area by usingcommunication channels of the communication channel groups, said mobileunit comprising:an operation circuit configured to designate the baseunit or a fixed unit as a destination of communication connection; acontrol circuit for sequentially receiving signals transmitted viacommunication channels included in communication channel groups assignedto cells of the cellular communication system, identifying, on the basisof the reception results, the one of the communication channel groupswhich is assigned to the one of the cells in which the mobile unit issituated, selecting, on the basis of the identification results, thoseof the communication channel groups which are not assigned to the onecell in which the mobile unit is situated, and storing the selectedcommunication channel groups; a receiving circuit configured toselectively receive, in accordance with a designated result of theoperation circuit, a signal sent via a communication channel of thecommunication channel group assigned to the cell in which the mobileunit is located or a signal sent from the fixed unit via a communicationchannel of one of the communication channel groups stored in the controlcircuit; and a transmission circuit configured to perform, in accordancewith the designated result of the operation circuit, transmission in afirst case of performing radio communication with the base unit andtransmission in a second case of performing communication with the fixedunit.
 5. The mobile unit according to claim 4, wherein said radiocommunication between said base unit and said mobile unit is performedat a level in a first transmission output level range.
 6. The mobileunit according to claim 5, wherein said transmission circuit performstransmission at a level in the first transmission output level range inthe first case of performing radio communication with the base unit andtransmission at a level which is lower than the output levels in thefirst transmission output level range in the second case of performingcommunication with the fixed unit.
 7. A fixed unit situated in aphysical area of a cellular communication system wherein the physicalarea is divided into a plurality of cells, each cell comprising a baseunit and being assigned one of a plurality of communication channelgroups employed in the cellular communication system, such that at leastthose of the communication channel groups which are assigned to adjacentones of the cells are different from each other, and radio communicationis performed between the base units of the cells and a mobile unitmoving in the physical area by using communication channels of thecommunication channel groups, said fixed unit comprising:a wiredcommunication circuit configured for signal transmission/reception witha communication network via a wired line; a radio communication circuitfor sequentially receiving signals transmitted via communicationchannels included in communication channel groups employed in thecellular communication system, identifying, on the basis of thereception results, the one of the communication channel groups which isassigned to the one of the cells in which a fixed unit is situated,selecting, on the basis of the identification results, those of thecommunication channel groups which are not assigned to the one cell inwhich the fixed unit is situated, and storing the selected communicationchannel groups; and a communication control circuit configured toconnect the wired communication circuit and the radio communicationcircuit in a case of performing communication between the communicationnetwork and the mobile unit.
 8. A mobile unit of a cellularcommunication system used in a mobile communication system comprisingthe cellular communication system wherein a physical area is dividedinto a plurality of cells, each cell comprising a base unit and beingassigned one of a plurality of communication channel groups employed inthe cellular communication system, such that at least those of thecommunication channel groups which are assigned to adjacent one of thecells are different from each other, and radio communication isperformed between the base units of the cells and the mobile unit movingin the physical area, and a fixed unit situated in said physical area ofthe cellular communication system and connected to a communicationnetwork via a wired line, said mobile unit comprising;a connectiondestination determining circuit configured to determine the base unit orthe fixed unit as a destination of radio connection; a first radiocommunication circuit configured to perform radio communication with thebase unit of one of the cells, when the base unit is determined as thedestination of radio connection by the connection destinationdetermining circuit, by selectively using a communication channel of thecommunication channel group assigned to the cell in which the mobileunit is located; a control circuit for sequentially receiving signalstransmitted via communication channels included in communication channelgroups assigned to cells of the cellular communication system,identifying, on the basis of the reception results, the one of thecommunication channel groups which is assigned to the one of the cellsin which the mobile unit is situated, selecting, on the basis of theidentification results, those of the communication channel groups whichare not assigned to the one cell in which the mobile unit is situated,and storing the selected communication channel groups: and a secondradio communication circuit configured to perform radio communicationwith the fixed unit, when the fixed unit is determined as thedestination of radio connection by the connection destinationdetermining circuit, by selectively using a communication channel of oneof the communication channel groups stored in the control circuit. 9.The mobile communication system according to claim 8, wherein saidconnection destination determining circuit determines the base unit orthe fixed unit as the destination of radio connection, on the basis ofinformation for designating the connection destination input by anoperation circuit.
 10. The mobile communication system according toclaim 8, wherein said connection destination determining circuitincludes a circuit configured to determine whether an associated mobileunit is present in such a position as to permit radio communication withthe fixed unit, and the connection destination determining circuitdetermines the fixed unit as the radio connection destination if saidcircuit has determined that the associated mobile unit is present insuch a position as to permit radio communication with the fixed unit,and determines the base unit as the radio connection destination inanother case.
 11. The mobile unit according to claim 8, wherein saidradio communication between said base unit and said mobile unit isperformed at a transmission output level in a first transmission outputlevel range.
 12. The mobile unit according to claim 11, wherein saidradio communication between said mobile unit and said fixed unit isperformed at a transmission output level which is lower than the outputlevels in the first transmission output level range.
 13. A mobilecommunication system comprising:a cellular communication system coveringa physical area which is divided into a plurality of cells, each cellcomprising a base unit and being assigned one of a plurality ofcommunication channel groups employed in the mobile communicationsystem, such that at least those of the communication channel groupswhich are assigned to the adjacent ones of the cells are different fromeach other; a communication network; a mobile unit movable in thephysical area and configured to communicate with a base unit of a cellin which the mobile unit is located using a communication channel in thecommunication channel group associated with the cell in which the mobileunit is located; and a fixed unit connection to the communicationnetwork, wherein said fixed unit comprises:a control circuit forsequentially receiving signals transmitted via communication channelsincluded in communication channel groups assigned to cells of thecellular communication system, identifying, on the basis of thereception results, the one of the communication channel groups which isassigned to the one of the cells in which a fixed unit is situated,selecting, on the basis of the identification results, those of thecommunication channel groups which are not assigned to the one cell inwhich the fixed unit is situated, and storing the selected communicationchannel groups; and a radio communication circuit configured to select acommunication channel to be used, on the basis of the communicationchannel groups stored in said control circuit, and to perform signaltransmission/reception with the mobile unit via the selectedcommunication channel.
 14. The mobile communication system of claim 13,wherein the fixed unit is connected to the communication network via awired line.
 15. A system comprising a fixed unit connected to acommunication network and a mobile unit configured to communicate with abase unit of a cell in which the mobile unit is located, the cell beingin a cellular communication system having a plurality of cells whereinsaid fixed unit comprises:a control circuit for sequentially receivingsignals transmitted via communication channels included in communicationchannel groups assigned to cells of the cellular communication system,identifying, on the basis of the reception results, the one of thecommunication channel groups which is assigned to the one of the cellsin which a fixed unit is situated, selecting, on the basis of theidentification results, those of the communication channel groups whichare not assigned to the one cell in which the fixed unit is situated,and storing the selected communication channel groups; and a radiocommunication circuit configured to select a communication channel to beused, on the basis of the communication channel groups stored in saidcontrol circuit, and to perform signal transmission/reception with themobile unit via the selected communication channel.
 16. The apparatus ofclaim 15, wherein the fixed unit is connected to the communicationnetwork via a wired line.
 17. A mobile communication system comprising:acellular communication system for covering a geographical area which isgeographically divided into a plurality of cells, each cell comprising abase unit and being assigned one of a plurality of communication channelgroups employed in said cellular communication system, such that atleast those of the communication channel groups which are assigned toadjacent ones of the cells are different from each other, wherein radiocommunication is performed between the base unit, and a mobile unitmoving in the geographical area, using a communication channel includedin the one of the communication channel groups and a first transmissionoutput level; and a fixed unit connected to a communication network viaa wired line, wherein said mobile unit and said fixed unitcomprise:identification means for sequentially receiving communicationchannels included in the communication channel groups employed in saidcellular communication system, and identifying, on the basis of thereception results, the one of the communication channel groups which isassigned to the one of the cells in which the mobile unit is situated;storage means for selecting, on the basis of the identification results,those of the communication channel groups employed in said cellularcommunication system which are not assigned to the one of the cells atwhich the mobile unit is situated, and storing the selectedcommunication channel groups; and means for performing radiocommunication between the mobile unit and the fixed unit, using acommunication channel included in the communication channel groupsstored in the storage means, and a second transmission output levellower than the first transmission output level.
 18. A mobilecommunication system comprising:a cellular communication system forcovering a geographical area which is geographically divided into aplurality of cells, each cell comprising a base unit and being assignedone of a plurality of communication channel groups employed in saidcellular communication system, such that at least those of thecommunication channel groups which are assigned to adjacent ones of thecells are different from each other, wherein radio communication isperformed between the base unit and a mobile unit moving in thegeographical area, using a communication channel included in the one ofthe communication channel groups and a first transmission output level;and a fixed unit connected to a communication network via a wired line,wherein said mobile unit and said fixed unit comprise:identificationmeans for sequentially receiving communication channels included in thecommunication channel groups employed in said cellular communicationsystem, and identifying, on the basis of the reception results, the oneof the communication channel groups which is assigned to the one of thecells in which the mobile unit is situated; storage means for selecting,on the basis of the identification results, those of the communicationchannel groups employed in said cellular communication system which arenot assigned to the one of the cells at which the mobile unit issituated, and storing the selected communication channel groups; andmeans for performing radio communication between the mobile unit and thefixed unit, using a communication channel included in the communicationchannel groups stored in the storage means.
 19. The mobile communicationsystem according to claim 18, wherein said fixed unit is located in acell of the cellular communication system and comprises:a firstreceiving circuit configured to receive signals sent via thecommunication channels of the communication channel groups assigned tothe cells of the cellular communication system; a control circuitconfigured to determine whether communication channels of thecommunication channel groups other than the communication channel groupassigned to the cell in which the fixed unit is located are available onthe basis of the signals received by the first receiving circuit; and asecond receiving circuit configured to receive a signal transmitted fromthe mobile unit via a communication channel determined to be availableby the control circuit.
 20. The mobile communication system according toclaim 18, wherein said mobile unit comprises:a receiving circuitconfigured to receive, in a first time period, a signal sent via acommunication channel of the communication channel group of the cell inwhich the mobile unit is located, and receiving, in a second time perioddifferent from the first time period, a signal sent via a communicationchannel of one of the communication channel groups other than thecommunication channel group assigned to the cell in which the mobileunit is located; and a call reception detection circuit configured todetect a call coming via the base unit and a call coming via the fixedunit on the basis of the signals received by the receiving circuit. 21.The mobile communication system according to claim 18, wherein saidmobile unit comprises:an operation circuit configured to designate thebase unit or the fixed unit as a destination of radio connection; and aradio communication circuit configured to selectively perform a radiocommunication operation with the base unit and a radio communicationoperation with the fixed unit in accordance with a designated contentinput by the operation circuit.
 22. The mobile communication systemaccording to claim 18, wherein said fixed unit is located in a cell ofthe cellular communication system and comprises:a first receivingcircuit configured to selectively receive signals sent via thecommunication channels of the communication channel groups assigned tothe cells of the cellular communication system; a control circuitconfigured to determine whether communication channels of thecommunication channel groups other than the communication channel groupassigned to the cell in which the fixed unit is located are available onthe basis of the signal received by the first receiving circuit; and atransmission circuit configured to transmit a signal to the mobile unitvia a communication channel determined to be available by the controlcircuit; and said mobile unit comprises: a receiving circuit configuredto receive the signal from the fixed unit via the communication channeldetermined to be available by the control circuit of the fixed unit; amode setting circuit configured to set a cordless mode in a time periodin which the signal transmitted to the mobile unit is received by thereceiving circuit, and to set a cellular mode in another time period;and a radio communication circuit configured to selectively perform aradio communication operation with one of the base units and a radiocommunication operation with the fixed unit in accordance with the modeset by the mode setting circuit.